A method of this type is known from DE 100 51 462 A1. The known method involves a method for correction of the beam hardening for images which have been recorded within the framework of computer tomography.
Computer tomography (CT) is an established imaging method of x-ray diagnostics. The latest CT systems are multilayer CT, spiral CT and conebeam CT with flat-panel detector. The x-ray radiation required for computer tomography is created for current devices with the aid of x-ray tubes. The radiation of x-ray tubes is polychromatic. The consequences of this are as follows:
During the penetration of material the lower-energy photons are attenuated more strongly than the photons of higher energy, which leads to a material-dependent and wavelength-dependent beam hardening. The result is a dominance of photons of higher energies in the spectrum. This phenomenon even occurs with objects made of homogeneous material. For the penetration of a cylindrical body filled with water transverse to the longitudinal axis the hardening for rays at the edge is less than for rays in the middle of the cylinder which travel a long way through the body.
The theory of CT reconstruction algorithms requires monochromatic radiation. If the polychromasy is ignored the reconstruction leads to what is known as a cupping effect: the reconstructed attenuation coefficient (gray value) decreases continuously from the edge inwards. This effect is relatively easy to correct for so-called water-equivalent materials of a low ordinal number, such as soft tissue, fat and many plastics. The correction is undertaken in such cases within the context of a so-called water correction or 1st-order hardening correction.
Above and beyond this the beam hardening is increased by the presence of materials of higher ordinal number, above all for contrast media, but also for bones or with metal implants. Even after previous water correction local density faults still occur after the reconstruction, especially dark bars or shadow-type artefacts, for example between or in the extension of contrast media-filled vessels or heavily-absorbent bone structures. Such 2nd-order hardening artefacts can in extreme cases reach a strength of more than around 100 HU (1 HU=1 Hounsfield Unit). The 2nd-order hardening artefacts can adversely affect diagnosis. For example the risk arises of false positive findings of pseudo-stenoses which are apparent constrictions in what are actually normal vessels, or the detectability of smaller lesions in cases of density caused by hardening is rendered more difficult. The cause of the 2nd-order artefacts is the energy dependence of the attenuation coefficients which differ greatly from water in materials with a higher ordinal number.
Water-equivalent materials are also referred to below as W-materials and materials with a higher ordinal number, for example contrast media, implants or bones, as K-materials.
Correction methods require knowledge of which of the individual measured values have been influenced by the transmission through K-material and how many of these the respective measurement rays have penetrated.
The known correction method obtains the knowledge from a first image or volume reconstruction about which of the individual measured values have been influenced by transmission through K-material and how much K-material the respective measurement ray has penetrated. Subsequently a segmentation of the reconstructed volume is conducted with a separation into two components, namely W-material and K-material generally being undertaken with the aid of a threshold criterion. By a subsequent reprojection, in which the individual measurement rays are traced back through the volume, it can be approximately determined which material lengths the individual measurement rays have traveled through W-material and through K-material respectively. After the hardening correction at least one second reconstruction is necessary, with further iteration steps able to follow. This type of correction method is often also referred to as bone correction.
On account of the reprojection and the second reconstruction such correction methods are complex.
A direct method for correction of the projection data would be desirable, so that the expensive iteration with reprojection and second reconstruction can be avoided.